Our transportation fuels and many chemical products including solvents, fertilizers, pesticides, and plastics are derived from petroleum. This dependence on petroleum has probably the biggest impact on our unsustainable way of living. Atmospheric CO2 is at the highest recorded level and it is predicted to further increase in the foreseeable future. Emerging serious environmental issues caused by an accelerated global climate change and ocean acidification are unavoidable if we keep using petroleum at the current rate. In addition, petroleum reservoirs in general and in particular with cheap mining access are going to decline. To ensure the future advancement of human society, there is an ever-increasing demand for renewable biofuels and bioenergy products as an alternative to fossil fuels and petroleum.
Production of cost-competitive fuels and chemicals by microbial fermentation using renewable feedstock is a desirable alternative. Plant biomass especially agricultural residues, such as corn stover and sugarcane bagasse, represents an important feedstock as they are renewable and they do not compete with food production. Sugar content in many types of agricultural residues is about 60-70% of the dry weight, which is comparable to corn. However, utilization of sugars from plant biomass in a cost-effective manner remains a challenge. Unlike starch, woody biomass (both cellulosic and lignocellulosic biomass), including plant biomass, is naturally resistant to deconstruction. Crystalline fibers of cellulose are encased in a covalently linked mesh of lignin and hemicellulose. D-glucose is the subunit of cellulose and xylose is the main component of hemicellulose (20-40% of biomass dry weight). While glucose can be metabolized very efficiently by many common industrial hosts, such as baker's yeast, industrial microbes have either no native xylose catabolism pathway or very low efficient pathways. Furthermore, the presence of glucose prevents the consumption of other sugars like xylose in microbes, a well-described phenomenon called catabolite repression.
Accordingly, there is a need for improving the derivation of cost-competitive fuels and chemicals from woody biomass using industrial microbes. In particular, mechanisms to counteract the wild type industrial microbes' natural inhibition of efficient metabolism using woody biomass as feedstock are needed.